This invention is related to a method of producing intermediates for zwitterionic siloxane polymers having a controlled polymer structure. More particularly, this invention relates to a method of producing zwitterionic siloxane polymer precursors utilizing hydrolyzed difunctional alkoxysilanes and aryloxysilanes in a polymerization reaction with a hydroxy-terminated siloxane oligomer.
The zwitterions on the siloxane polymers provide ionic cross-linking between the siloxane polymers due to the coulombic forces exerted by the ions. An example of an ionic cross-link which may exist between two siloxane polymer segments is illustrated in the following formula: ##STR1## wherein R' is a divalent hydrocarbon radical of from 1 to 20 carbon atoms and R is a divalent hydrocarbon radical of from 2 to 20 carbon atoms.
These cross-links reduce the mobility of the polymer segments and increases their stiffness. For example, polydimethylsiloxanes (DP=500) are typically liquid at room temperature, yet corresponding zwitterionic polysiloxanes are solid rubbers at this temperature. Introducing zwitterions to as few as 1% of the silicone atoms within a siloxane fluid will provide a solid elastomeric material.
These elastomeric materials exhibit high adhesion to glass and other substrates such as, for example, wood, metal, polycarbonates, polystyrene, polyphenylene oxides and blends thereof, etc. The elastomeric properties and adhesive properties of these zwitterionic siloxanes make them suitable for use as adhesives, elastomeric adhesives, sealants, coatings, injection moldable and compression moldable rubbers and plastics, and various silicone based rubbers.
The synthesis of zwitterionic siloxane polymers has been described by Graiver et al. in J. Polymer Sci., Vol. 17, page 3559 (1979). Graiver et al. utilize aminoalkyl-dimethoxysilanes with low molecular weight polydimethylsiloxane oligomers (degree of polymerization, DP, approximately 30). Graiver et al. disclose that successful copolymerization is obtained from these substituents when producing low molecular weight copolymers (DP approximately 500). These low molecular weight copolymers have a lower viscosity than desired and provide poor zwitterionic siloxane rubbers when allowed to react with .delta.-propane sultone (see Example III).
High molecular weight aminoalkyl-siloxane polymers are difficult to obtain reproducibly from the copolymerization of aminoalkyl-dimethoxysilanes and hydroxy-terminated polydimethylsiloxane oligomers by the method described by Graiver et al. It is difficult to obtain high molecular weight aminoalkyl siloxane polymers where the difunctional aminoalkyl-silane has alkoxy or aryloxy functional groups.
It has been discovered that prehydrolysis of these functional groups of an aminoalkyl-silane provides improved incorporation of said silane into siloxane polymers. Aminoalkyl-siloxane polymers of a high molecular weight are easily obtained from the copolymerization of hydroxy-terminated polydimethylsiloxane oligomers and hydrolyzed aminoalkyl-dimethoxysilanes. Furthermore, it has been discovered that the use of hydroxy-terminated polydimethylsiloxanes of random length provide zwitterionic siloxane polymers with superior properties.